The present invention relates to a poly(amide-ether-imide) and the preparation of the same, and in particular relates to a poly(amide-ether-imide) prepared by subjecting a cyclized imide-containing dicarboxylic acid to polycondensation with an aromatic diamine.
Poly(amide-imide)s are characterised as high performance polymeric materials having both superior heat resistance of polyimide and superior toughness, and processability of polyamide. The most economic and simple method for the preparation of poly(amide-imide) is by the use of trimellitic anhydride (TMA). TMA has the characteristics of both anhydride and carboxylic acid, hence when it is subjected to polycondensation with diamines, poly(amide-imide)s are formed. However, difficulties are encountered when it is desired to prepare linear poly(amide-imide)s with high molecular weight from TMA and diamine. In general, poly(amide-imide)s are prepared by first activating TMA to 4-chloroformyl phthalic anhydride, then subjecting the anhydride to low temperature solution polycondensation with diamine to form poly(amide-amic acid), and followed by cyclizing the amic acid by the film-baking method or high temperature ring-closing method, or the addition of condensing agents, such as acetic acid anhydride. However, there are several problems with the above-mentioned method which are described as follows.
(1) Activation of TMA increases the manufacturing cost and complicates the operation procedures. The activated TMA is unstable and is easily hydrated and degraded.
(2) As the polyamide-amic acid prepared by low temperature polycondensation contains strong acid (HCl), the amide bonds of the amic acid will break when subsequently subjected to cyclization by direct baking or by the addition of solvents at an elevated temperature. This breakage of amide bonds will cause the reduction of the molecular weight of the resultant products.
(3) The manufacturing cost will also increase if a treatment with acetic acid anhydride is used.
To overcome the above problems, U.S. Pat. Nos. 3,920,612 and 4,048,144 disclose a process comprising first subjecting TMA to polycondensation with amine to form diacid of imide, then activatiing the diacid to acid chloride, and followed by polycondensing the acid chloride at low temperature. However, high manufacturing cost is still a problem and the intermediate acid chloride is not easily purified.
Japan laid-open Patents Nos. Sho 44-19274, Sho 46-20068 and Sho 50-33120 describe a process for preparing a poly(amide-imide). The process includes reacting diisocyanates with TMA or its derivates in specific solvents and catalysts at an elevated temperature. As the source of diisocyanates is limited and the diisocyanates are not as stable as diamine in storage, this process is not satisfactory.
Japan laid-open Patent Nos. Sho 49-4077, Sho 58-180532 and British Patent No. 1383480 describe a poly(amide-imide) prepared by direct copolymerizing TMA and diamine in the presence of catalysts at a temperature of 200.degree. C. However, at such a temperaure, the polymer is easily coloured and a linear polymer is not necessarily formed.
Accordingly, a novel synthetic method of poly(amide-imide) was developed by the inventors in 1989, which method comprises first preparing an imide-containing dicarboxylic acid by thermocondensation, and then subjecting the acid to polycondensation with diamine directly by using a phosphite condensing agent. The manufacturing cost of this method is low and high molecular weight polymers are obtainable.